1. Field of the Invention
The present invention relates to a magnetic resonance imaging based on magnetic resonance phenomena exhibited by spins of nuclei. More particularly, this invention is concerned with medical-purpose magnetic resonance imaging method and magnetic resonance imaging system for executing imaging that uses mutual interaction between different kinds of pools of nuclear spins.
2. Description of the Related Art
Currently, a technique in which a "magnetization transfer" (MT) effect (or "magnetization transfer contrast" (MTC) effect) is utilized to produce a contrast image whose contrast resolution varies depending on whether a living body exerts the MT effect is known as one of various medical resonance imaging techniques. A practical example of this imaging technique has been disclosed in U.S. Pat. No. 5,050,609 ("Magnetization Transfer Contrast and Proton Relaxation and Use Thereof in Magnetic Resonance Imaging" filed by Robert S. Balanban et al.).
The MT effect originates from a saturation transfer (hereinafter ST) technique proposed by Forsen and Hoffman (Refer to the Journal of Chemical Physics, Vol.39 (11), pp.2892-2901 (1963) written by Forsen et al.), and is based on chemical exchange and/or cross relaxation between protons of a plurality of kinds of pools of nuclei, for example, free water and a macromolecule.
The relationship involving magnetic resonance between the protons of free water and those of a macromolecule is such that free water having a long transverse relaxation time T2 (T2=approx. 100 msec.) and the macromolecule having a short relaxation time T2 (T2=approx. 0.1 to 0.2 msec.) become resonant with the same frequency. The relationship involving frequency spectra between the free water and the macromolecule is shown on the left-hand column of FIG. 9A. The exchange and relaxation of magnetizations between them is shown on the right-hand column of FIG. 9A (the same applies to FIGS. 9B and 9C). As for a signal induced by the free water, since the free water has a long relaxation time T2, a signal subjected to Fourier transformation is, as illustrated, expressed as a wave having a sharp peak and a small half-width. By contrast, as for a signal induced by protons of a macromolecule such as protein whose relative motions are restricted, since the macromolecule has a short relaxation time T2, a signal subjected to Fourier transformation is expressed as a wave having a large half-width but not having a peak in the frequency spectrum.
In the known imaging method based on the MT effect, when the peak resonant frequency of the free water signal is regarded as a center frequency, as shown in FIG. 9B, a frequency-selective pre-pulse (MTC pulse) is used to excite the component of the free water having a frequency shifted by, for example, 500 Hz from the resonant frequency of the free water (off-resonance excitation). This causes the magnetization H.sub.f of the free water to transfer to the magnetization H.sub.r of a macromolecule. Consequently, as shown in FIG. 9C, the level of an MR signal induced by protons of the macromolecule is lowered, and the level of an MR signal induced by protons of the, free water is lowered at a higher rate. This means that there is a difference in signal level between a region on which chemical exchange and/or cross relaxation between free water and a macromolecule is reflected and a region on which the chemical exchange and/or cross relaxation is not reflected. Contrast images of different contrast resolutions can therefore be produced. This is usable in differentiating a lesion in a living body or the like from a normal tissue therein.
The known MT effect is based on a so-called "negative" transfer of a magnetization in which the magnetization H.sub.f of protons of free water is transferred to the magnetization H.sub.r of protons of a macromolecule through off-resonance excitation in order to lower an MR signal induced by the free water. As a result, the signal-to-noise ratio of the signal is low.
The foregoing MT effect results from an interaction between spins of a plurality of different pools of nuclei. Even imaging, which has been unconscious of any particular influence of the MT effect in the past, undergoes the influence of the MT effect in reality.
In particular, for example, in a fluid-attenuated inversion-recovery (FLAIR) or fast FLAIR, numerous applied inversion pulses (for example, 180.degree. RF pulses) bring about the MT effect relative to adjoining slices. This leads to a decrease in signal strength. Moreover, since such imaging techniques do not take account of the MT effect and an interpulse time between applied inversion pulses is often not uniform, irregular sensitivity occurs between slices due to the MT effect.
Further, a plurality of 180.degree. refocusing pulses employed in fast spin echo (FSE) imaging to be implemented in fast FLAIR imaging trigger the MT effect. This presumably deteriorates the contrast between, for example, white matter and gray matter.
Conversely, causing the MT effect can change image contrast. However, in imaging employing an IR-(inversion recovery) system pulse sequence, imaging techniques making use of the MT effect have not yet been developed.